A method is described as continuous production or endless rolling if a casting plant is connected to a rolling plant so that the slab cast in the die of a casting plant is fed directly—without being separated from the slab section just cast and without intermediate storage—into a rolling plant and is rolled there to a desired final thickness in each case. The beginning of the slab can thus already be rolled to its finished final thickness while the casting plant continues to cast the same slab, i.e. no end of the slab exists at all. This is also referred to as directly-coupled operation or endless operation of the casting and rolling plant.
In semi-continuous production or “semi-endless rolling” the cast slabs are separated after casting and the separated slabs are fed to the rolling plant without intermediate storage and cooling to ambient temperature.
The slab emerging from the die of the casting plant first passes through a slab-guiding device located immediately after the die. The slab-guiding device, also referred to as the “slab-guiding corset” comprises a number (usually three to six) guide segments, wherein each guide segment has one or more (usually three to ten) pairs of guide elements, preferably designed as slab support rollers. The support rollers are rotatable around an axis running orthogonally to the direction of transport of the slab.
Instead of being designed as slab-support rollers, individual guide elements can also be designed as static, e.g. skid-shaped components.
Regardless of the actual design of the guide elements, said elements are disposed on both sides of the slab width sides, so that the slab is guided by a series of upper and lower guide elements and conveyed to a roughing train.
Viewed in precise terms, the slab is not only supported by the slab-guiding device but also already by a lower end area of the die, which is why the die could also be seen as a part of the slab-guiding device.
The slab solidification begins at the upper end of the (through) die at the bath surface, the so-called “meniscus”, wherein the die is typically around 1 m long (0.3-1.5 m).
The slab emerges vertically downwards out of the die and is diverted into the horizontal. The slab-guiding device therefore essentially has a curved course over an angular range of 90°.
The slab emerging from the slab-guiding device is reduced in thickness in the roughing train (HRM, High-Reduction Mill), the intermediate strip produced here is heated up by a heating device and rolling is completed in a finish rolling train. In the finish rolling train the metal is hot-rolled, which means that the material to be rolled has a temperature during rolling of greater than its recrystallization temperature. With steel this ranges from above around 750° C., usually hot rolling takes place at temperatures of up to 1200° C.
During hot rolling of steel the metal is mostly in the austenitic state, where the iron atoms are disposed cubically face-centered. Rolling is then said to be in the austenitic state when both the starting and the end rolling temperature lie in the austenitic region of the respective steel. The austenitic region of a steel is dependent on the steel composition, but as a rule lies above 800° C.
Decisive parameters in the production process of hot steel strip from combined casting and rolling plants are the casting velocity with which the slab leaves the die (and passes through the slab-guiding device) as well as the mass throughput or volume flow, which is specified as the product of the casting velocity and the thickness of the slab and is usually expressed as the unit [mm*m/min].
The produced steel strips are further processed for motor vehicles, domestic appliances and the building industry.
The continuous and semi-continuous production of hot steel strips is already known from the prior art. As a result of the coupling of casting plant and rolling plant, dealing with all the plant parameters represents a high demand in process technology terms. Modifications in the casting and rolling process, especially through changes in the casting velocity in combination with the slab thickness, as well as a material-specific solidification coefficient which is able to be controlled by cooling, have a significant effect on the production quality and energy efficiency of the plant.
Generic methods or plants are known for example from EP 0 415 987 B1, EP 1 469 954 B1 and DE 10 2007 058 709 A1 and WO 2007/086088 A1.
Significant progress in hot rolling technology has been achieved in particular by Acciaieria Arvedi S.p.A. which has developed a thin slab endless method based on an ISP (In-line Strip Production) technology called Arvedi ESP (Endless Strip Production).
In this ESP method the casting and the rolling process are linked to each other in an especially advantageous manner so that a subsequent cold rolling is no longer required for many hot rolled steel qualities. With such hot rolled steel qualities, in which subsequent cold rolling continues to be required, the number of rolling stands can be reduced compared to conventional rolling trains.
An ESP plant disclosed for example at the Rolling & Processing Conference '08 (September) and installed in Cremona, Italy, for hot-rolled steel production operated by Arvedi, comprises a roughing train connected after the slab casting plant with three roughing stands, two strip separation facilities, an induction oven for intermediate heating of the rough-rolled intermediate strip followed by a finish rolling train with five finishing rolling stands. The endless strip emerging from the roughing train is cooled in a cooling section and wound by means of three underfloor coilers into strip rolls with a weight of up to 32 tons. Positioned before the underfloor coilers is a separation facility in the form of a rapid-cut shear. Depending on the steel types and the strength of the rolled steel strip, the production capacity of this single-Strand production line is around 2 million tons per year (mtpy). This plant is also described to some extent in the following publications: Hohenbichler et al: “Arvedi ESP—technology and plant design”, Millenium Steel 2010, Mar. 1, 2010, pages 82-88, London, and Siegl et al: “Arvedi ESP—First Tin Slab Endless Casting and Rolling Results”, 5th European Rolling Conference, London, Jun. 23, 2009.
However a particular disadvantage proves to be a 17 m slab support length, which is too short, that is any distance described more precisely as the “metallurgical length”, between the casting area of the die, in more precise terms between the bath level of the liquid steel referred to as the “meniscus” and the end of the slab-guiding device facing towards the roughing train.
As already described at the outset, the slab-guiding device forms a partly curved receiving shaft between the guide elements or the slab support rollers to accommodate the freshly cast slab (still having a liquid core).
Thus, in the present context, the active guide surface or outer line of the last guide element facing towards the roughing train or of the last support roller of the upper guide element series is to be understood as the end of the slab-guiding device.
As the distance from the meniscus increases, the slab guided in the slab-guiding device or the steel strip which is in its initial form cools down more and more. Each inner area of the slab, which is still liquid or is of a doughy-soggy consistency, is referred to below as the liquidus tip. A “liquidus tip” further from the die of the liquidus is defined as that central cross-sectional area of the slab in which the temperature just still corresponds essentially to the steel solidus temperature and subsequently falls below this. The temperature of the liquidus tip therefore corresponds to the solidus temperature of the respective sort of steel (typically between 1300° C. and 1535° C.)
The rolling of a completely solidified or cooler cast slab demands a significantly higher energy outlay than the rolling of a cast slab with a hot cross-sectional core.
For volume flows below 380-400 mm*m/min there has previously only been discontinuous production (batch operation) in the ISP or ESP method.
The CSP (Compact Strip Production) methods known from the prior art likewise operate at slab thicknesses of 45-65 mm with volume flows below around 400 mm*m/min using a roller hearth furnace with a length of 250 m and greater, wherein exclusively discontinuous manufacturing (batch operation) or semi-continuous manufacturing takes place. In the latter 3-6 separate slabs (no longer connected to the casting plant or the die) are rolled endlessly.
In EP 0 889 762 B1, for endless casting and rolling of a hot strip, a volume flow>0.487 mm2/min (converted to the conventional unit mentioned at the start: >487 mm*m/min) is proposed. Casting with such a high volume flow with comparatively small slab thickness however proves to be too fast for many sorts of steel to enable a sufficient manufacturing quality to be guaranteed.